The country was in a terrible position economically and involved in two wars, and the voters were finally beginning to realize who created that mess. By nominating two people unlikely to win the election, they could pass the problems off to the Democrats for four years. (I think the fact that they didn't try a last-minute "October surprise" of cooked-up scandal is significant. In addition, I don't recall many reports of voter harassment in the minority community or sneaky business with voting machines.) Then after obstructing and criticizing their way through Obama's first term, spinning everything as his fault, they thought they'd be in a good position to take over again in 2012.

If by some chance McCain and Palin had happened to win anyway, Republicans would have remained in control (probably with Cheney wielding a lot of power behind the scenes).